Some high fidelity audio systems, e.g., so-called “surround sound” systems using Dolby Pro-logic™ or Dolby Digital™ include power amplifiers for each of the sound signals provided by such Dolby™ systems, e.g., left, right, center, left rear, right rear. In a home audio system, these audio power amplifiers would be powered by a common main power supply.
Normally, for a comfortable listening level, one needs to boost the signal response in the low frequency bass range. The Dolby™ systems provide a sub-woofer signal which is outputted to a sub-woofer system, which can be a separate active unit which includes a preamplifier/buffer, a high power audio power amplifier, and a large diameter loudspeaker for providing a sufficient audio output at low bass frequencies below the frequencies of a regular woofer. The sub-woofer comes with its own dedicated audio power amplifier in order to boost the sound in the low frequency range to overcome a shortcoming in total sound quality because the sub-woofer bass sounds require a disproportionately large amount of power to be sufficiently heard due to the characteristics of the human ear response, the usually insufficient dimensions of the listening room, and because the sub-woofer loudspeaker, which has a much larger structure than other loudspeakers of a speaker system, requires more power just to move its large mass and the accordingly large amount of air moved by the speaker cone. This way, the dedicated sub-woofer power supply drives the sub-woofer and this large amount of power is not a load on the main amplifier power supply which powers the various other amplifiers thus permitting the other sounds to be fully reproduced.
However, the use of a separate active sub-woofer has a higher total system cost, and is prone to low frequency signal overload (too much bass) because it requires separate volume controls for the main unit and the sub-woofer. Thus, an increase adjustment of the main volume control can cause such bass overload. Power supplies are very expensive since they require a power transformer, rectifiers, and large filter capacitors. In order to save the considerable expense of providing a separate power supply for the high powered sub-woofer power amplifier, it is desirable for the sub-woofer power amplifier to share the same power supply with the main audio power amplifiers. Such an integrated system arrangement is used in RCA home theater model RT2250 made by Thomson Consumer Electronics Inc., Indianapolis, Ind., USA, which includes a dedicated sub-woofer power amplifier powered from the same power supply as the main audio amplifiers.
However, having accomplished this substantial cost reduction, we are again faced with the original problem of the drain of a large amount of power from the main power supply when the power is needed for the main power amplifiers. One solution is to make the main power supply much larger, but this is very expensive and adds considerable weight to the unit. Thus, such an approach is counterproductive.
Much of the time, the main power supply, e.g., having a 50 watt audio output capability, is only lightly loaded even if music is played “very loud”. In even large homes, playing at a one watt average output level would probably rattle windows, much less damage hearing. The reason for the desirability of high power output amplifiers (which do sound better) is so that the peaks of the signal are not clipped or distorted even if played at such a “very loud” sound level. Therefore, the high power capability of the main power supply for the main power amplifiers is mostly not used. Thus, powering the dedicated sub-woofer large power amplifier from the main power supply, including setting the maximum sub-woofer power to be more than the main channel power, would usually not be in conflict with the needs of the main power amplifiers. In this manner, the sub-woofer can deliver double power for low frequency components because in the normal case, the mid-range music content only delivers a small portion of the energy relative to the sub-woofer low frequency range. In this manner, the system can more efficiently utilize the whole power of the amplification system while greatly reducing costs.
Another problem is that with the sub-woofer system gain set for a disproportionally higher power output in order to compensate for the lesser audible effect on the listener in the sub-woofer audio frequencies, as discussed above, the sub-woofer system output can easily reach full output with the sub-woofer sound signal being “clipped”, or with the sub-woofer loudspeaker reaching its physical limits and distorting the reproduced sub-woofer sound.
U.S. patents where an electronic action is initiated in response to a signal and in connection with sub-woofers are: U.S. Pat. No. 6,026,168 wherein level adjustment is made to sub-woofer signals in response to the volume of a scaled main signal; U.S. Pat. No. 5,636,288 wherein the AC power to a separate integral power amplifier of a sub-woofer system is shut-off in response to detection of an on-off signal; and U.S. Pat. No. 5,347,230 wherein an auxiliary power source is activated in response to the level of an audio signal so that the output power capability and power dissipation of the output transistors are reduced at low signal levels and higher at high signal levels, with different parameters being used to reduce ripple voltage in connection with sub-woofer frequencies.